Fluids in oil production are comprised of oil, gas and water. Reservoir water can be water that is very salty containing many elements. In particular, at high temperature, the production water can contain elements such as iron, zinc or lead, coming from the rocks with which the reservoir water has been in contact for millions of years. When the gas associated with the production contains sulphides such as hydrogen sulphide, iron, zinc and/or lead sulphides can then be formed. These salts can be deposited in the immediate vicinity of the reservoir or in the production installations such as in the pipes (“tubing”). Such deposits have in particular been observed during the concomitant production of gas and of brine in the Gulf of Mexico and in the British sector of the North Sea. In order to remove them, it is necessary to clean the equipment on a regular basis, which has a negative effect on the productivity of the method.
A solution for removing these deposits consists in carrying out acid washings. In addition to their short duration of action, these treatments however are not without risk for man and the equipment, in particular in conditions of high pressure and of high temperature.
In order to overcome this problem, it is possible to inhibit the formation of the deposits by injecting inhibitors continuously, at the bottom of the well, when the installations are in place. Otherwise, the injection of the product is carried out using an injection technique called “squeeze”. This technique consists in injecting a large quantity of product into the oil reservoir while production is stopped. The deposit inhibitor injected into the reservoir must be absorbed by the rock during the injection and be released progressively when production resumes, in order to prevent the formation of deposits over a long period of time, during the production of reservoir water.
Among the solutions proposed in prior art for inhibiting the formation of sulphide deposits, it has been suggested to add to the drilling fluid injected into the well an inhibiting agent able to prevent the formation of sulphide crystals, to prevent their growth or to disperse them. As such, a polymer of low molecular weight, which is a homo- or copolymer of vinyl sulphonate, was studied by M. M. JORDAN et al. in document SPE 64427 presented at the SPE Asia Pacific Conference in Brisbane (Australia), on 16 Oct. 2000. Other inhibitors have been proposed in documents U.S. Pat. No. 7,159,655 and U.S. Pat. No. 7,398,824. This entails respectively acrylamide, quaternary ammonium and possibly acrylate copolymers, and acrylamide, diallyldimethylammonium salt and possibly acrylate copolymers, which are introduced into a carrier fluid or into the brine. It is indicated that these polymers are more soluble, in high-density brines, than polymers made from acrylamidomethylpropane sulphonic acid, maleic acid and acrylic acid. Document GB-2 448 442 reports the introduction of the same type of polymer into a fracturing fluid. Document U.S. Pat. No. 5,171,459 suggests the use of alkyldiphenylether sulphonates such as the Dowfax product from DOW CHEMICAL. It has been shown that these compounds disperse the sulphide deposits better than other sulphide polymers such as a poly(vinyl sulphonate) and a vinylsulphonate/styrene/maleic anhydrous copolymer.
It was however observed that the known inhibitors of sulphide deposits were not effective for use in conditions of high pressure and high temperature. The same applies to phosphonates which are known to break down at high temperature, with the degradation products of these compounds furthermore having very low solubility in the presence of alkaline earth metal cations, which substantially reduces their use and makes it even practically impossible in reservoir at more than 150° C.
There is therefore still a need to have a compound that makes it possible to inhibit or to slow the formation of sulphide deposits, in particular zinc, iron and lead sulphides, which is stable at high temperature and high pressure and which can also be effective against the formation of other inorganic deposits or compatible with the inhibitors used for this purpose. It would furthermore be desirable that this compound can be released in a prolonged manner.
The inventors have demonstrated that these needs could be satisfied by using, as a sulphide deposit inhibitor, a particular copolymer such as a styrene sulphonic acid/anhydrous maleic copolymer. It has already been suggested to use such a copolymer as an inhibitor of non-sulphide inorganic deposits, made from calcium carbonate and from barium sulphate, in oil wells (FR 2 803 304). These deposits are generally formed during the placing into contact of the brine present in the reservoir with the fluid injected in order to recover the oil. It is commonly acknowledged that calcium sulphate or carbonate deposit inhibitors are not suitable for preventing the formation of sulphide deposits. This results in the fact that the calcium carbonate and barium sulphate bear positive surface charges, contrary to lead, iron and zinc sulphides of which the isoelectric point is less than 4 (M. Kosmulski, Journal of Colloid and Interface Science, 35, (2011), 1-15). The anionic polymers used to inhibit the formation of the first ones are therefore not adapted to develop electrostatic interactions with the second ones. As such, it was not foreseeable that the aforementioned copolymers could offer a solution to the aforementioned needs.